(1) Field of the Invention
The present invention relates to a plastic optical fiber, more particularly to a plastic optical fiber having a core-cladding structure and exhibiting excellent light transmission properties.
(2) Description of the Prior Arts
Improvements in the light transmission properties of optical fibers has led to their increased use in the fields of optical communication and the like.
Conventional optical fibers include those having a core consisting of a glass and those having a core consisting of a plastic material. The conventional glass core optical fibers exhibit excellent light transmission properties and, therefore, are useful in long distance communication. However, this type of optical fiber exhibits poor flexibility and, therefore, easily breaks.
Plastic core optical fibers on the other hand can transmit light only a relatively short distance. However, this type of optical fiber exhibits excellent flexibility and, thus, is very useful for short distance communication devices.
Conventional plastic core optical fibers are mainly of a step index type. A step index type optical fiber is composed of a core comprising a polymer of a high refractive index and a cladding comprising a polymer of a low refractive index. When light is applied to the step index type optical fiber, the light is transmitted through the core and is reflected entirely on the interface between the core and the cladding. In this case, it is important that the cladding be firmly bonded to the core and that the cladding be highly resistant to formation of cracks therein.
Japanese Patent Publication No. 43-8978 discloses an optical fiber comprising a core consisting of polystyrene and a cladding consisting of polymethylmethacrylate. The bonding property of polystyrene to polymethylmethacrylate, however, is unsatisfactory. Therefore, the resultant optical fiber has unsatisfactory optical properties at the interface between the core and the cladding.
Another type of optical plastic fiber, as disclosed in several references, comprises a core consisting of polymethylmethacrylate, exhibitng a high transparency, and a cladding consisting of a fluorine-containing polymer.
For example, Japanese Patent Publication Nos. 43-8973, 56-8321, 56-8322, and 56-8323 disclose plastic optical fibers comprising a core consisting of polymethylmethacrylate and a cladding consisting of a polymer of polyfluorinated alkyl methacrylate. The polyfluorinated alkyl methacrylate polymer, however, exhibits a poor bonding property to the polymethylmethacrylate core and poor resistance to thermal decomposition thereof, and therefore, is unsatisfactory as a material for forming the cladding.
In still another type of plastic optical fiber, the core consists of polymethylmethacrylate and the cladding consists of a copolymer of vinylidene fluoride with tetrafluoroethylene. In this optical fiber, the cladding can be firmly bonded to the core. However, since the vinylidene fluoride-tetrafluoroethylene copolymer is highly crystalline, the light transmitted through the core is scattered at the interface between the core and the cladding. This scattering phenomenon results in decreased light transmission.
The properties required for a polymer in plastic optical fibers are non-absorption non-scattering of light irradiated thereto, a low refractive index, and an excellent bonding property to the core. Polyfluorinated alkyl methacrylates exbibit a low refractive index and excellent transparency, but are unsatisfactory in resistance to thermal decomposition. Accordingly, conventional cladding polymers are not always satisfactory in some of the above-mentioned properties.